As someone currently working on an IMAP client, my strategy has been to start by implementing the base IMAPrev2 specification and checking that it still works with a rev1 server. RFC 9051 has guidance on how to handle servers that use older IMAP protocol versions and is largely backwards compatible.
The goal is to have the client work without any extensions. E.g., if the server doesn’t have the UNSELECT capability it falls back to selecting a nonexistent mailbox (which has the same effect). Obviously there are some very useful extensions that the client needs to support, but those can be viewed as optimizations to reduce network traffic or add extra features if the server supports them.
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As someone currently working on an IMAP client, my strategy has been to start by implementing the base IMAPrev2 specification and checking that it still works with a rev1 server. RFC 9051 has guidance on how to handle servers that use older IMAP protocol versions and is largely backwards compatible.
The goal is to have the client work without any extensions. E.g., if the server doesn’t have the UNSELECT capability it falls back to selecting a nonexistent mailbox (which has the same effect). Obviously there are some very useful extensions that the client needs to support, but those can be viewed as optimizations to reduce network traffic or add extra features if the server supports them.